In general, session refers to a frame of communication between two medium. A PHP session is used to store data on a server rather than the computer of the user. Session identifiers or SID is a unique number which is used to identify every user in a session based environment. The SID is used to link the user with his information on the server like posts, emails etc.
Before you can store any information in session variables, you must first start up the session. To begin a new session, simply call the PHP session_start() function. It will create a new session and generate a unique session ID for the user.
The PHP code in the example below simply starts a new session
The session_start() function first checks to see if a session already exists by looking for the presence of a session ID. If it finds one, i.e. if the session is already started, it sets up the session variables and if doesn't, it starts a new session by creating a new session ID.
You must call the session_start() function at the beginning of the page i.e. before any output generated by your script in the browser, much like you do while setting the cookies with setcookie() function.
Storing and Accessing Session Data
You can store all your session data as key-value pairs in the $_SESSION superglobal array. The stored data can be accessed during lifetime of a session. Consider the following script, which creates a new session and registers two session variables.
To access the session data we set on our previous example from any other page on the same web domain — simply recreate the session by calling session_start() and then pass the corresponding key to the $_SESSION associative array.
The PHP code in the example above produce the following output.
Hi, John Doe
If you want to remove certain session data, simply unset the corresponding key of the $_SESSION associative array, as shown in the following example:
However, to destroy a session completely, simply call the session_destroy() function. This function does not need any argument and a single call destroys all the session data.
Every PHP session has a timeout value — a duration, measured in seconds — which determines how long a session should remain alive in the absence of any user activity. You can adjust this timeout duration by changing the value of session.gc_maxlifetime variable in the PHP configuration file (php.ini).
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